In an automatic power transmission for an automotive vehicle, the gear ratios in the forward and reverse drive ranges of the transmission are selected by selectively actuating fluid operated frictional units including, for example, a high-and-reverse clutch, a forward drive clutch, a brake band and a low-and-reverse brake which are all operated by fluid pressure delivered from a hydraulic control system. In a known power transmission using these frictional units, the forward drive clutch in particular is maintained coupled throughout the conditions in which an automatic forward drive range is established in the transmission. When forward drive clutch alone is permitted to be operative, the first or "low" gear ratio in the automatic forward drive range is established in the transmission mechanism. If the brake band is applied with the forward drive clutch maintained in the coupled condition, then a shift is made in the transmission mechanism from the first gear ratio to the second gear ratio in the automatic forward drive range. If, furthermore, the brake band is released and the high-and-reverse clutch in turn is made operative with the forward drive clutch held coupled, there is produced in the transmission mechanism an upshift from the second gear ratio to the third or "high" gear ratio in the automatic forward drive range. When, on the other hand, both of the high-and-reverse clutch and the low-and-reverse brake are made operative, a reverse drive gear position is selected in the transmission mechanism.
Each of the frictional units thus operative is actuated to couple or apply by a control fluid pressure, unusally called line pressure, developed by a pressure regulator valve incorporated in the hydraulic control system. If the line pressure is supplied at an uncontrolled rate to any one or two of these fluid operated frictional units during shifting from the neutral gear position to the automatic forward drive range position or the reverse drive gear position, mechanical shocks tend to be created in the transmission mechanism and will give the occupants of the vehicle an unpleasant sensation.
The brake band to produce the second forward drive gear ratio is automatically operated to apply or release by means of a fluid operated band servo unit depending upon the vehicle speed under automatic forward drive range condition. If the line pressure is supplied to the band servo unit at an uncontrolled rate during shifting from the first gear ratio to the second gear ratio in the automatic forward drive range, mechanical shocks are also produced in the transmission mechanism.
In order to alleviate these shocks to be produced during selection of the automatic forward drive range position or the reverse drive gear position or upshifting between the gear ratios in automatic forward drive range, there is proposed and put to practical use a hydraulic transmission control system having incorporated therein a plurality of pressure accumulator units each of which is arranged to lessen the shocks to be produced during each of the shifts from the neutral gear position to the automatic forward drive gear position and the reverse drive gear position and the upshifts between the gear ratios in the automatic forward drive range. Installation of such a plurality of pressure accumulator units results in an enlarged and intricate construction of the control valve assembly incorporating the hydraulic transmission control system and further in an increased production cost of the transmission system as a whole.
The present invention contemplates elimination or reduction of the mechanical shocks usually concommitant with the selection of the automatic forward drive range and reverse drive gear position and the upshifting between the gear ratios in the automatic forward drive range by means of a shock control arrangement using a single pressure accumulator unit so as to provide compact and small sized construction of the transmission control valve assembly without having recourse to increasing the production cost of the transmission system having the shock control arrangement.